So sounds like the concensus is, use TH where
something is purely a heading and use TD where
something is functioning both as a heading and as
data. And you can use the header attribute in both
tags.

Another question:
Can you have a header AND an ID both in one TD tag? In
other words, can you say that a TD tag is serving as a
heading (ID attribute) for other data cells, but its
own data also belongs under a heading somewhere else
in the table (header attribute)?